#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>

#define PORT 1234
#define BACKLOG 5
#define MAXDATASIZE 1000
void process_cli(int  connfd, struct sockaddr_in client);

main()
{
int  listenfd, connfd;
pid_t  pid;
struct  sockaddr_in  server;
struct sockaddr_in  client;
int  len;

if ((listenfd =socket(AF_INET, SOCK_STREAM, 0)) == -1) {
perror("Creatingsocket failed.");
exit(1);
}

int opt =SO_REUSEADDR;
setsockopt(listenfd,SOL_SOCKET, SO_REUSEADDR, &opt, sizeof(opt));
bzero(&server,sizeof(server));
server.sin_family=AF_INET;
server.sin_port=htons(PORT);
server.sin_addr.s_addr= htonl (INADDR_ANY);
if (bind(listenfd,(struct sockaddr *)&server, sizeof(server)) == -1) {
perror("Bind()error.");
exit(1);
}

if(listen(listenfd,BACKLOG)== -1){
perror("listen() error\n");
exit(1);
}
len=sizeof(client);

while(1)
{
if ((connfd =accept(listenfd,(struct sockaddr *)&client,&len))==-1) {
perror("accept() error\n");
exit(1);
}
if ((pid=fork())>0){
close(connfd);
continue;
}
else if (pid==0) {
close(listenfd);
process_cli(connfd, client);
exit(0);
}
else {
printf("fork()error\n");
exit(0);
}
}
close(listenfd);
}

void process_cli(int connfd, struct sockaddr_in client)
{
int num;
char  recvbuf[MAXDATASIZE], sendbuf[MAXDATASIZE], cli_name[MAXDATASIZE];
printf("Yougot a connection from %s. ",inet_ntoa(client.sin_addr) );
num = recv(connfd,cli_name, MAXDATASIZE,0);
if (num == 0)
{
close(connfd);
printf("Client disconnected.\n");
return;
}
cli_name[num - 1] ='\0';
printf("Client'sname is %s.\n",cli_name);

while (num =recv(connfd, recvbuf, MAXDATASIZE,0)) {
recvbuf[num] ='\0';
printf("Receivedclient( %s ) message: %s",cli_name, recvbuf);
int i = 0;
for (i = 0;i < num - 1; i++) {
if((recvbuf[i]>='a'&&recvbuf[i]<='z')||(recvbuf[i]>='A'&&recvbuf[i]<='Z'))
{
recvbuf[i]=recvbuf[i]+ 3;
if((recvbuf[i]>'Z'&&recvbuf[i]<='Z'+3)||(recvbuf[i]>'z'))
recvbuf[i]=recvbuf[i]- 26;
}
sendbuf[i] =recvbuf[i];
}
sendbuf[num - 1]= '\0';

send(connfd,sendbuf,strlen(sendbuf),0);
}
close(connfd);
}
